The present invention relates to hydrostatic transaxle assemblies, and more particularly, to such assemblies which have the capability for the operator to select from among various operating modes.
Certain vehicles, such as lawn and garden tractors, and smaller farm tractors, have used hydrostatic transaxle assemblies for transmitting engine torque to a pair of ground-engaging drive wheels to propel the vehicle. A typical transaxle assembly which has been used commercially included a variable displacement hydraulic pump, such that the ratio of pump output flow to pump input speed (engine speed) could be infinitely varied by the vehicle operator. This ability to infinitely vary the output-input ratio, without interrupting torque transmission, makes transaxle assemblies greatly preferred over conventional clutch and gear transmission arrangements for vehicles of the type referred to above.
Vehicles of the type referred to above which utilize transaxle assemblies frequently encounter operating conditions in which one of the drive wheels has less traction than the other drive wheel, and it has long been recognized by the manufacturers of such vehicles that it is desirable to provide the operator of the vehicle with some type of control arrangement for effectively dealing with such loss of traction situations. At the same time, however, it is recognized that under most operating conditions, the vehicle must be able to have "normal differentiating action" between the two drive wheels, i.e., it must be possible for the two drive wheels to be driven at substantially different speeds, such as during a sharp turn.
In addition to dealing with the loss of traction situation, there are various other operating conditions which occur with such vehicles. For example, such vehicles are frequently towed a substantial distance, such as to a particular work site, and if the transaxle is left in its normal operating mode during towing, the result can be excessive wear of internal parts, and overheating of rapidly circulating hydraulic fluid, especially if the vehicle is being towed at a speed faster than its normal driving speed. In addition, it is normally desirable for transaxle assemblies on vehicles of this type to have the capability of both low-speed, high-torque operation (working mode) and high-speed, low-torque operation (transport mode).